1. Field of the Invention
The present invention relates to an anode including an anode current collector and an anode active material layer arranged on the anode current collector, and a secondary battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as camera-integrated VTRs (videotape recorders), cellular phones, or laptop computers are widely used, and size and weight reduction in the portable electronic devices and an increase in longevity of the portable electronic devices have been strongly demanded. Accordingly, as power sources for the portable electronic devices, the development of batteries, specifically lightweight secondary batteries capable of obtaining a high energy density have been promoted. Among them, a secondary battery (a so-called lithium-ion secondary battery) using insertion and extraction of lithium for charge-discharge reaction holds great promise, because the secondary battery is capable of obtaining larger energy density, compared to a lead-acid battery or a nickel-cadmium battery.
The lithium-ion secondary battery includes a cathode, an anode and an electrolytic solution, and the anode has a configuration in which an anode active material layer including an anode active material is arranged on an anode current collector. As the anode active material, a carbon material is widely used; however, recently with enhancement of performance and expansion of functions in portable electronic devices, a further improvement in battery capacity is desired, so it is considered to use silicon instead of a carbon material. It is because the theoretical capacity of silicon (4199 mAh/g) is much larger than the theoretical capacity of graphite (372 mAh/g), so an increase in battery capacity is expected.
However, when silicon is deposited as the anode active material by a vapor-phase method, a large number of pores are formed in the anode active material to increase the surface area of the anode active material. In this case, the anode active material has high activity, so an electrolytic solution is easily decomposed during charge and discharge, and lithium is easily inactivated. Thereby, while the secondary battery obtains a higher capacity, cycle characteristics which are important characteristics of the secondary battery easily decline.
Therefore, to improve cycle characteristics even in the case where silicon is used as the anode active material, various ideas have been made.
More specifically, a technique in which in the case where the deposition of a silicon thin film is performed a plurality of times by a vapor-phase method, ions are applied to a surface of the silicon thin film before the second or later deposition steps (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-293899), a technique in which an anode current collector having a three-dimensional configuration such as foam metal or a sintered fiber metal body is used (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-071305), or a technique in which silicon is sintered to be integrated with an anode current collector (for example, refer to Japanese Unexamined Patent Application Publication Nos. H11-339777 and H11-339778) or the like has been proposed.
Moreover, a technique in which silicon particles are coated with a fired body (ceramic) such as a metal oxide (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2004-335334 and 2004-335335), a technique in which an oxide layer such as silicon oxide is formed on a surface of a silicon alloy layer (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-319469), a technique in which a conductive metal is reductively deposited on silicon powder (for example, refer to Japanese Unexamined Patent Application Publication No. H11-297311), a technique in which silicon compound particles are coated with a metal (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-036323), a technique in which a metal element not alloyed with lithium is dispersed in silicon particles (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-273892), a technique in which copper is dissolved into a silicon thin film (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-289177) or the like has been proposed.